1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medicine images of a patient's body organs of interest. In particular, the present invention relates to a novel procedure and system for accurately detecting the occurrence of valid scintillation events.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
Emitted gamma photons are typically detected by placing a scintillator over the region of interest. Such scintillators are conventionally made of crystalline material such as NaI(TI), which interacts with absorbed gamma photons to produce flashes of visible light. The light photons emitted from the scintillator crystal are in turn detected by photosensor devices that are optically coupled to the scintillator crystal, such as photomultiplier tubes. The photosensor devices convert the received light photons into electrical pulses whose magnitude corresponds to the amount of light photons impinging on the photosensitive area of the photosensor device.
Not all gamma interactions in a scintillator crystal can be used to construct an image of the target object. Some of the interactions may be caused by gamma photons that were scattered or changed in direction of travel from their original trajectory. Thus, one conventional method that has been used to test the validity of a scintillation event is to compare the total energy of the scintillation event against an energy “window” or range of expected energies for valid (i.e., unscattered) events. In order to obtain the total energy of the event, light pulse detection voltage signals generated from each photosensor device as a result of a single gamma interaction must be accurately integrated from the start of each pulse, and then added together to form an energy signal associated with a particular event. Energy signals falling within the predetermined energy window are considered to correspond to valid events, while energy signals falling outside of the energy window are considered to correspond to scattered, or invalid, events, and the associated event is consequently not used in the construction of the radiation image, but is discarded. Without accurate detection of the start of an event, the total energy value may not be accurate, which would cause the signal to fall outside of the energy window and thereby undesirably discard a useful valid event.
Another instance of inaccurate information may arise when two gamma photons interact with the scintillation crystal within a time interval that is shorter than the time resolution of the system (in other words the amount of time required for a light event to decay sufficiently such that the system can process a subsequent light event as an independent event). The finite time resolution of the system arises because the response of scintillation crystals is not strictly linear, especially when there are large changes (e.g., a factor of approximately 100 or greater) in the intensity of radiation incident on the crystal. If the crystal has been operating under a high “count rate” or level of incident radiation (e.g. 106 photons/sec for a gamma ray source), and this incident radiation is suddenly reduced to a lower count rate (e.g. 104 photons/sec for a gamma ray source), the light output of the crystal does not immediately fall off. Instead, there is a slow decay in the light output of the crystal due to residual phosphorescence (so-called “afterglow”), so that accurate measurements cannot be made until the crystal “afterglow” has disappeared. Because of this effect, light events from the two gamma interactions are said to “pile up,” or be superposed on each other. The signal resulting from a pulse pile-up would be meaningless, as it would not be possible to know whether the pulse resulted from two valid events, two invalid events, or one valid event and one invalid event.
Different solutions to the pulse pile-up problem are known in the prior art. One such solution involves the use of pile-up rejection circuitry, which either precludes the detector from processing any new pulses before processing has been completed on a prior pulse, or stops all processing when a pile-up condition has been identified. This technique addresses the problem of post-pulse pile-up, wherein a subsequent pulse occurs before processing of a pulse of interest is completed. Such rejection circuitry, however, may undesirably increase the “deadtime” of the imaging system, during which valid gamma events are being received but are not able to be processed, thereby undesirably increasing the amount of time needed to complete an imaging procedure. The “deadtime” of a system also may be a function of the speed limitations of the signal processing circuitry.
Another known technique addresses the problem of pre-pulse pile-up, wherein a pulse of interest is overlapped by the “afterglow” (e.g. the trailing edge or tail) of a preceding pulse. This technique uses an approximation of the preceding pulse tail to correct the subsequent pulse of interest. Such approximation is less than optimal because it is not accurate over the entire possible range of pile-up conditions. Further, it requires knowledge as to the precise time of occurrence of the preceding pulse, which is difficult to obtain using analog signals. Additionally, this technique consumes a large amount of computational capacity.
Yet another problem encountered in the conventional detection and processing of valid light events is the effect of signal noise on accurate event location processing. In particular, direct current (DC) drifts or other sources of noise may alter the signals from the photosensor devices significantly enough to cause the calculation of the spatial location of an event to be unacceptably inaccurate.
A known prior art solution to this problem is disclosed in commonly assigned U.S. Pat. No. 5,847,395, incorporated by reference herein in its entirety. The '395 patent discloses the use of a flash analog-to-digital converter (FADC) associated with each photosensor device (e.g., photomultiplier tube (PMT)) and a data processor that integrates the FADC output signals, generates a fraction of a running sum of output signals, and subtracts the fraction from the integrated output signals to generate an adjustment signal to correct the output signals for baseline drifts. However, this solution does not address the pile-up problem as it is concerned with energy-independent locational computation.
Therefore, there exists a need in the art for a solution that addresses the problem of pulse pile-up.